20．Living in the wild can be hard．Finding food and staying safe aren't easy．Each day，animals struggle to survive in their habitats．Not all animals get by on their own．Some animals form a close partnership with other kinds of animals．These pairings are called symbiotic （共生的） relationships．
In a symbiotic relationship，the animals depend on each other．One animal helps the other meet its needs．Sounds good，right？Not always．Some animals are not very kind to their partners．In some cases，one animal meets its needs but hurts its partner．Take ticks，for example．These insects suck blood to live．To get blood，they attach themselves to other kinds of animals．Ticks can pass germs that cause disease instead of helping their hosts．
In other relationships，animals don't treat their partners so poorly Both animals benefit from living with the other animal．
Small animals called cleaner shrimps have found a way of helping fish at coral reefs．As their name suggests，the shrimps clean the fish．They hang out at what scientists call a cleaning station．A fish stops by．Then a shrimp climbs onto the fish and even steps into the fish's mouth．The shrimp uses its tiny claws to pick stuff off the fish's body．That can include dead skin，tiny pieces of food，and wee creatures that can hurt the fish．The fish gets a nice cleaning．The shrimp enjoys a tasty meal offish trash．
Small birds called plovers are also in the cleaning business．They have big customers-crocodiles．Crocs have long snouts（鼻子）filled with sharp teeth，Cleaning them is tricky．When a croc opens its mouth，the plover hops right in．The croc does not snap its snout shut．Instead，it lets the plover eat small，harmful animals attached to its teeth．The plover gets an easy meal while the croc gets clean teeth．
9．The text is mainly aboutB．
A．the hard life in the wild
B．animals in a symbiotic relationship
C．animals living together
D．some dangerous animals
10．According to the text，in a symbiotic relationship，D．
A．animals depend on each other
B．birds depend on animals for food
C．both sides can get help
D．not all animals treat their partners well
11．Why do the cleaner shrimps clean the fish？A
A．To get fish trash for food．
B．To eat wee creatures．
C．To help fish go out of coral reefs．
D．To get food for fish．
12．According to the text，it can be inferred thatB．
A．living in the wild can be hard
B．different animals have different living ways
C．crocodiles mainly live on plovers
D．ticks are useful to some animals．
分析 本文主要介绍了生物界存在的共生现象．在自然界中，不同的动物有不同的生存方式，有些生物之间存在一种合作关系，他们相互依存，各取所需．而有的是单方面获利，而且并不是所有的动物都对它们的共生伙伴友好．
解答 9--12 BDAB
9．B．主旨大意题．根据"Some animals form a close partnership with other kinds of animals．These pairings are called symbiotic （共生的） relationships．"并联系上下文可知本文主要介绍了生物界的一种共生现象，很多生物之间存在相互依存、相互帮助的关系．故选B．
10．D．细节理解题．根据第二段"Take ticks，for example．These insects suck blood to live．To get blood，they attach themselves to other kinds of animals．Ticks can pass germs that cause disease instead of helping their hosts．"可知并不是所有的动物都对它们的共生伙伴很友好．壁虱就是这样的例子．故选D．
11．A．细节理解题．根据"The fish gets a nice cleaning．The shrimp enjoys a tasty meal offish trash．"可知清洁虾清洗鱼是为了把鱼身上的垃圾作为食物．故选A．
12．B．推理判断题．根据"Not all animals get by on their own．Some animals form a close partnership with other kinds of animals．These pairings are called symbiotic （共生的） relationships．"可知有些动物是靠自己去捕捉食物，有些动物是靠共生来获取食物，也就是说不同的动物有不同的生存方式．故选B．
点评 此题主要考查学生的细节理解和推理判断能力．做细节理解题时要结合原文和提干有针对性的找出相关语句进行仔细分析，再结合选项选出正确答案．在做推理判断题时不要以个人的主观想象代替文章的事实，要根据文章事实进行合乎逻辑的推理判断．做主旨大意题时考生首先要仔细阅读短文，完整了解信息，准确把握作者观点，确定主旨．
